Analysis of Swelling Force of Lithium-ion Power Battery

1. Author Information and Article Abstract

In 2020, Central South University and CATL jointly studied the cyclic swelling force changes of the ternary system power battery under different design and assembly process conditions, and further combined with 3D simulation to simulate and analyze the swelling force of the battery pack on the module shell after the cycle failure. To guide battery design.

2. Test Information

Single Battery: S40 (containing 40 electrode), S60 (containing 60 electrode), the gap between the electrode and the aluminum shell is the same.

Modular Battery: S40_1P6S (including 6 S40 single batteries), S60_1P4S (including 4 S60 single batteries).

2.1 Test Parameters: 25℃, 1C/1C.

3. Analysis of Results

3.1 Analysis of the Cyclic Swelling Force of Measured Single Cells and Modules

Judging from the capacity retention rate and swelling force curve of the single cell, there is no significant difference between the S40 and S60 cells in the first 1000 cycles, indicating that there is a gap between the electrode sheet and the aluminum shell before that. It is enough to support the swelling of the electrode sheet. When the cycle continues to 1300 cycles, the capacity decay rate of the S60 battery is significantly greater than that of the S40 battery, and the increase rate of the swelling force of the S60 is also greater than that of the S40.This shows that when the gap between the electrode and the aluminum shell in the square aluminum shell cell is constant, the cell with more electrode pieces will increase the swelling force faster at the end of the long-term cycle, which will accelerate the battery capacity decay.

Two modular batteries, the gap between the single cells of S40_1P6S is 2.4mm, and the gap between the single cells of S60_1P4S is 2.0mm. Judging from the capacity retention rate and swelling force curves of the two modular batteries, there is no significant difference between the S40 and S60 cells in the first 400 cycles. Explain that before this, the gap reserved between the battery cell and the module housing is still enough to support the swelling of the battery cell. When the cycle continues to 1000 cycles, the capacity decay rate of the S60_1P4S module is significantly greater than that of the S40_1P6S module, and the increase rate of the swelling force of the S60 is also greater than that of the S40. There are two main reasons: one is that the reserved space inside the S60_1P4S module is smaller than that of the S40_1P6S module, so the housing of the module will expand earlier. The other reason is that the S60 single cell battery is due to the large number of electrode pieces, compared with the S40 single cell, the swelling force will be greater, so the swelling force fed back to the module housing is also greater. Therefore, in the module design, the influence of the reserved gap and the swelling force on the attenuation of the battery capacity should be considered in a balanced manner.

3.2 Simulation Analysis of Module Design

Based on the previous actual swelling force test of single cell and modular battery, the simulation method is used to analyze the maximum swelling force that the three main structural parts of the module (top cover, side plate, top side plate welding point) can withstand, in advance evaluate the reliability of the module structure. Using about simulation software, input some measured cell expansion force and material characteristics and other parameters, and finally output the stress that the structural components of the module bear, as shown in Figure 5 and Table 2. Comparing the simulated stress values and failure thresholds of different components given in Table 2, the swelling force of each component of the S60_1P4S module is greater than that of the S40_1P6S module. If the failure threshold of the component is less than the simulated value, it means that the structural component does not meet the design requirements.

Figure 5. Module swelling force simulation (a-c) S40 1P6S module simulation and (d-f) S60 1P4S module simulation.

4. Summary

This paper uses experimental testing and 3D simulation to analyze the swelling force of the ternary system single battery and module cycle to EOL, and points out the internal reserved gaps, the number of electrode sheets, and the failure stress threshold of the module structure that need to be paid attention to in the battery design. The influence of other parameters on the battery cycle.

5. Reference

Yongkun Li, Chuang Wei, Yumao Sheng, Feipeng Jiao, and Kai Wu. Swelling Force in Lithium-Ion Power Batteries，Ind. Eng.Chem. Res，2020, 59, 27, 12313–12318.

IEST Related Test Instruments Recommendations:

SWE series in-situ swelling analysis system (IEST):

1. Integration of multiple in-situ cell characterization methods (stress & swelling thickness): Measure the swelling thickness and swelling force during the charging and discharging process of the cell at the same time, and quantify the changes in the swelling thickness and swelling force of the cell;

2. More detailed and stable testing system: utilizing a highly stable and reliable automated regulation platform, equipped with high precision thickness measurement sensors and pressure regulation system, the relative thickness measurement resolution is 0.1µm, realizing the long cycle monitoring of the long-term charging and discharging process of the battery cell;

3. Diversified environmental control and testing functions: SWE series equipment can adjust the temperature of the charging and discharging environment, which is helpful for the study of the swelling behavior of the electric core under high and low temperature conditions; in addition to the conventional thickness and pressure testing, it can also realize the testing of parameters such as the swelling force of the electric core, the compression modulus, and the compression rate.